System for changing a rotational speed signal

ABSTRACT

The invention is based on a system for changing a signal representing rotational motion in a motor vehicle with first means for generating a first signal representing the rotational motion and a second means for generating at least one second signal representing additional information (direction of rotation, air gap, and/or brake lining wear). In addition, a third means is provided, by means of which the first signal (rotational speed signal) can be changed as a function of the second signal (direction of rotation, air gap, and/or brake lining wear). The essence of the invention is that the first means is designed in such a way that the first signal (rotational speed signal) assumes at least two first current values and/or two first voltage values. In addition, the third means is designed in accordance with the invention in such a way that, to change the first signal (rotational speed signal), at least one of the first current values and/or voltage values can be changed at least for a certain period of time into at least one second current value and/or voltage value as a function of the second signal (direction or rotation, air gap and/or brake lining wear).

BACKGROUND OF THE INVENTION

The invention is based on a system for changing a signal representingthe rotational speed of at least one, wheel of a motor vehicle. For theclosed-loop or open-loop control of the braking force, drive power,and/or dynamics of motion of a motor vehicle, it is known that therotational speeds of the wheels of the vehicle can be measured. Thestate of the art provides various methods (e.g., Hall ormagnetoresistive sensors) for this purpose. In addition, it is knownthat the point at which the brake lining of a vehicle brake becomes wornout can be determined by, for example, embedding contact pins a certaindepth below the surface of the brake lining. When the brake lining hasbeen worn down to this depth, the pins trigger a contact.

Active sensors for use in open-loop or closed-loop antilock, drive slip,engine, and transmission control systems in motor vehicles are describedin, for example, the article “Integrierte Hall-Effekt-Sensoren zurPositions-und Drehzahlerkennung”, eletronik industrie, Vol. 7, pp.29-31, 1995. In a two-wire system, sensors of this type supply twocurrent levels, which are converted by a precision resistor into twovoltage levels in a corresponding control device.

In addition to the Hall-effect sensors mentioned above, it is alsopossible to use magnetoresistive sensors to detect rotational speeds.This is known from, for example, the article “Neue, Alternative Lösungenfür Drehzahlsensoren im Kraftfahrzeug auf magnetoresisitiver Basis”,VDI-Berichte, No. 509, 1984.

A special shared device for detecting brake lining wear and therotational speed of a wheel is described In DE-C2 26 06 012 (U.S. Pat.No. 4,076,440). For this purpose, the detected brake lining wear and thewheel speed detected by means of an inductive sensor are sent over acommon signal line to an evaluation unit. This is achieved in that thewheel speed sensor is entirely or partially short-circuited in reactionto the detected amount of brake lining wear.

To detect the rotational speed of the wheel and the brake lining wear atthe wheel brake, other systems, such as those described in DE-C 43 22440, require at least two signal lines between the wheel unit and theevaluation unit.

In regard to the detection of rotational speed described above, it isknown that the air gap between the rotating toothed wheel rim and theactual sensor element has a considerable effect on the quality of therotational speed signal. In reference to this point, see, for example,DE-OS 32 01 811, for example.

In addition, in the case of systems to help the driver get the vehiclestarted (so-called “hill holders”), for example, information on therotational direction of the wheels is also required. Here it isespecially necessary to known if the vehicle is moving backwards. See,for example, DE-OS 35 10 651 on this point.

The information cited above and other types of data (such as data on thedegree of brake lining wear, the size of the air gap, and the directionof rotation) are usually detected close to the wheel and evaluated in acontrol unit located some distance away from the wheel. The informationmust therefore be transmitted to the control unit.

In the case of an engine (internal combustion engine or electric motor),it is known that the rpm's of the engine can be detected by means ofinductive, magnetoresistive, or Hall sensors.

The task of the present invention is to provide means for transmittingthe rotational speed signal and additional information in the simplestpossible, reliable manner.

BRIEF SUMMARY OF THE INVENTION

The invention is based on a system for changing a signal representing arotational movement with a first means for generating a first signalrepresenting the rotational movement and a second means for generatingat least one second signal representing an additional type ofinformation. Such information can consist in, for example, thedetermination of the direction in which rotation is occurring and/or ofthe size of the above-cited air gap and/or of the degree of the brakelining wear in at least one brake of the vehicle. The air gap can bedetermined on the basis of the amplitude of a signal associated with therotational speed signal. A third means is also provided, by means ofwhich the first signal (speed signal) can be changed as a function ofthe second signal (direction of rotation, air gap, and/or brake liningwear).

The core of the invention now consists in that the first means isdesigned in such a way that the first signal (rotational speed signal)assumes at least two first current values and/or two first voltagevalues. In addition, the third means according to the invention isdesigned in such a way that, to change the first signal (rotationalspeed signal), at least one of the first current values and/or voltagevalues can be changed for at least a certain period of time to at leastone second current value and/or voltage value as a function of thesecond signal (direction of rotation, air gap, and/or brake liningwear).

The invention offers the advantage that, in a simple and reliablemanner, the additional information concerning direction of rotation, airgap, brake lining wear, and/or other operating conditions of thevehicle, of the vehicle brake, and/or of the vehicle engine can betransmitted over the output line of the rotational speed sensor. As aresult, it is possible to eliminate, for example, the above-mentionedsecond signal line for the exclusive transmission of the additionalinformation.

Another advantage of the invention consists in that the speed sensor andthe detector of the above-cited additional information form a compactunit.

The system according to the invention is used advantageously in a motorvehicle, where the first signal can represent the rotational speed of avehicle wheel; the rpm's of the vehicle's engine, which can be either agasoline or diesel engine or an electric motor; and/or the rpm's of ashaft operationally connected to the vehicle's transmission.

In a variant, the invention is used in a wheel speed sensor unit, suchas that used in conjunction with an antilock, drive slip, and/orautomatic driving dynamics control system. Here the wheel speedinformation can be sent together with at least one of the additionaltypes of information cited above (direction of rotation, air gap, and/orbrake lining wear) from a sensor unit located close to the wheel to acontrol unit located some distance away from the wheel in a low-cost andreliable manner.

Another variant of the invention pertains to the possibility of usingthe system according to the invention to detect the rpm's of the engine.In this application, too, the engine rpm information can be senttogether with at least one of the above-mentioned additional types ofinformation from a sensor unit near the engine to a control unit somedistance away from the engine in a low-cost and reliable manner. Theadditional information to be transmitted in this case includes inparticular information on the backwards rotation of the engine. Aninternal combustion engine turns backwards primarily when the engine isbeing started and when it is stalling. With the systems being used atpresent, so-called intake manifold “bangs” can occur. An engine controlunit with conventional speed sensors (which do not recognize backwardsrotation) continues to receive a speed signal when the engine isrotating backwards, but it cannot tell that the engine is turning in thewrong direction. Because the engine is turning in the wrong direction,the ignition angle will be off by a wide margin the next time aninjection or ignition is initiated. If the fuel intake valve is openwhen ignition occurs, the above-mentioned intake manifold “bangs” willoccur. These bangs in the intake manifold can lead to the destruction ofthe following components:

the idling regulator,

the throttle valve,

the intake manifold itself, and

possibly the pressure gauge or the known hot-film air mass flow meter.

If it is possible to detect when the engine is turning backwards, itwill be possible to avoid bangs in the intake manifold by preventingignition from occurring. In this way, it will be is possible in turn toprevent the destruction of the above-mentioned components in a reliablemanner. It is also possible that, in cases where the system according tothe invention is used, these components could be designed to meet lessstringent requirements. For example, the intake manifold could be madeof plastic, which would lead to savings in both cost and weight.

The cost of modifying conventional systems in the manner required by thesystem according to the invention is relatively small, because the onlyitems which must be added to the conventional sensor are a rotationaldirection evaluator and a relatively simple logic circuit. The inputcircuit of the control device according to the invention must bemodified in such a way that the signal voltage can be evaluated withrespect to two different thresholds. With respect to geometry and theinstallation location, no changes or restrictions in comparison toconventional sensors without rotational direction detection areinvolved.

The first means according to the invention can be designed as an activerotational speed sensor, and the third means can be designed in such away that, to change the first signal, at least one of the first currentvalues is increased for at least a certain period of time to a secondcurrent value as a function of the second signal.

In an advantageous embodiment of the invention, the third means forchanging the current values has a current source. In particular, it isprovided that the third means has switching means for turning on and offthe superimposition of the first current values, at least two of whichare present, onto a current induced by the current source. In thisembodiment, therefore, it is provided that an additional current sourceis present in the rotational speed sensor, this current source beingactivated (or deactivated) by the presence of certain types ofinformation (e.g., backwards rotation, a certain degree of brake liningwear, too large an air gap). When this additional current source isactivated, at least one of the current levels which represent therotational speed to be detected is raised. The additional current sourcecan be integrated into the sensor or designed as a separate component.

The increase in level can involve both current levels; this isequivalent to an offset on the entire rotational speed signal, theoffset being a function of the rotational direction, the air gap, and/orthe brake lining wear. In particular, however, it is provided that onlyone of the two current levels is given an offset as indicated above.

The additional current source is usually activated by switching means,which superimposes the current levels representing the rotational speedthe current pf this additional current source. But it can also beprovided that the additional current source is turned on or off as afunction of the presence of backwards rotation, an excessive degree ofbrake lining wear, and/or an excessive air gap.

The additional current source can be activated as a result of a firstswitch and a second switch, this second switch preferably being designedas a transistor. In cases where excessive brake lining wear is beingdetected, it is advantageous for the first switch to be installed nearthe brake lining and for the second switch to be provided near therotational speed sensor. This embodiment is especially advantageous whenthe additional current source forms a structural unit with the speedsensor known in and of itself in either an integrated or separatedesign. The switching status of the second switch can be changed in amanner known in and of itself in that, for example, contact pins areembedded a certain depth below the surface of the brake lining. Acontact is triggered when the brake lining has been worn down to thedepth of the pins.

In this embodiment, it can be provided that the switching status of thefirst switch depends on the degree of brake lining wear and that theswitching status of the second switch depends on the switching status ofthe first switch.

The active rotational speed sensor can be designed in such a way that ithas two current sources to generate the first current values, at leasttwo of which are possible.

It is advantageous, furthermore, to provide transmission means fortransmitting the first signal or the changed first signal to theevaluation means (control unit). Conversion means can be provided in theevaluation means to convert the current values to the correspondingvoltage values.

For the evaluation of the signals sent to the control unit, at least onethreshold comparison can be provided in the evaluation means, by meansof which the current values or the corresponding voltage values arecompared with at least one threshold value. As a function of the resultof this comparison, it is then possible to drive display means, whichshow when an excessive air gap and/or excessive brake lining wear ispresent.

In a highly advantageous embodiment of the invention, it is providedthat the third means is designed so that the time at which the increasedepending on the second signal occurs is determined in such a way thatat least one of the two first current values is increased when thiscurrent value has occurred a defined number of times. This definedfrequency can be selected differently as a function of the informationto be transmitted.

In particular, in this last-mentioned embodiment, it can be providedthat the first means is designed in such a way that the first signal(rotational speed signal) periodically assumes the two first currentvalues, and that at least one of the two first current values isincreased when the first signal (rotational speed signal) has assumedthis current value n times, where n stands for a number greater than orequal to one.

The last-mentioned embodiment of the invention relates to theabove-mentioned offset on only one of the two current levels. Thisvariant has the advantage that, in comparison to the above-describedinformation-dependent offset on the entire rotational speed signal,here, only one of the two current levels, usually the high level, of therotational speed signal is increased in the presence of a certain typeof information (backwards rotation, an air gap large enough or brakelining wear significant enough to warrant a warning message). As aresult, the overall signal with the information pertaining to therotational speed and the other information can be evaluated more easilyin the control unit.

In addition to the data-dependent increase of every high level of therotational speed signal, it can also be provided in particular that onlyevery n-th high level is increased in an data-dependent manner. Thus, itcan be provided, for example, that, when backwards rotation has beendetected, every 2nd high level (n=2) is increased; when an excessive airgap is detected, every 4th high level (n=4) is increased; and/or whenexcessive brake lining wear is detected, every 8th high level (n=8) isincreased.

This can be achieved by means of counters, which, when a certain type ofinformation is present, such as a degree of braking lining wear whichwarrants a warning message, impose an offset depending on, for example,the brake lining wear only on every n-th high signal. In regard to thetransmission of the brake lining wear, furthermore, this variant of theinvention has the advantage that any possible rebounding of the brakelining wear switch will not result in an incorrect display, because theoffset is initiated only after n high levels have occurred.

In both variants (offset on the entire rotational speed signal or offseton the n-th high level), it can be provided that the change in the firstsignal (rotational speed) occurs as a function of a signal representingthe temperature of the vehicle brake and/or of the rotational speedsensor. The change in the first signal (rotational speed), furthermore,can also be actuated as a function of a signal representing the supplyvoltage of the rotational speed sensor.

It envisioned in particular here that, when the temperature of thesensor unit exceeds a certain critical value and/or when the supplyvoltage falls below a certain critical value, a display of theinformation which might be false under the circumstances such asinformation concerning brake lining wear is prevented. The temperatureof the vehicle brake and/or of the rotational speed sensor or theundervoltage is detected in this case by appropriate sensors or circuitsand comparators.

The second variant (offset on the n-th high level) also offers anotheradvantage, namely, that any rebounding of the above-mentionedcomparators or sensor circuits which may occur does not result in anincorrect display, because the offset is initiated only after n highlevels have occurred. As a result of the second variant, theadvantageous result is achieved that there is no longer any need forhysteresis behavior as a countermeasure against such rebounding.

As a result of the second variant, furthermore, there is no longer anyneed for hysteresis, which, while the vehicle is stopped or while theengine is off, prevents the data-dependent offset, especially the offsetrelevant to brake wear, from being permanently turned on or off, whichwould cause the (wheel) rotational speed evaluator to believe that awheel was rotating when it in fact was not.

A possible additional variant of the invention consists in the use ofthe system according to the invention in an automatic vehicle control,in which the suspension system (springs, shock absorbers) of the wheelunits are adjusted. In this case, the important input signals which aresensed are usually those which indicate the relative motions between thevehicle body and the wheels, i.e., the so-called “spring deflections”(spring deflection distance, spring deflection speed). In this case, itis also necessary to know, for example, the direction of the motion(compression or tension stage of the shock absorbers/springs). Accordingto the invention, a first means for generating a first signalrepresenting the relative motion and a second means for generating atleast one second signal representing additional information areprovided. The second means can be designed in such a way that thegenerated second signal contains information on whether the vehicle bodyis moving toward or away from the wheel unit. In addition, a third meansis designed in such a way that the first signal can be changed as afunction of the second signal.

The essence of this variant consists in that the first means is designedin such a way that the first signal assumes at least two first currentvalues and/or at least two first voltage values. The third means is thendesigned in such a way that, to change the first signal, at least one ofthe first current values and/or at least one of the first voltage valuescan be changed at least for a certain period of time to a second currentvalue and/or to a second voltage value as a function of the secondsignal.

Additional advantageous embodiments can be derived from the subclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a functional block diagram as known accordingto the state of the art. FIG. 2 shows a simple combination of an activerotational speed sensor with brake lining wear detection. FIGS. 3a and 3b show a circuit design of a first design variant of the systemaccording to the invention in the case where brake lining wear isdetected. FIG. 4 shows the courses of the associated signals. A seconddesign variant of the system according to the invention in the case ofbrake lining detection is shown in FIGS. 5 and 6. FIGS. 6a and 6 b showthe signals courses associated with the designated circuits when theswitch is closed and opened, respectively. FIGS. 7, 9 a, and 9 b show byway of example how an excessive air gap and the rotational direction ofthe wheel are detected, the associated signals being shown in FIGS. 8,10 a, and 10 b. FIGS. 11 and 12 show additions to the circuit designsshown in FIGS. 5 and 3b, while the signal courses are shown in FIG. 13.FIGS. 14 and 15 show an additional design variant of the invention, thecourses of the signals being shown in FIG. 16.

DETAILED DESCRIPTION OF THE INVENTION

The invention is to be described in detail on the basis of the forms ofembodiments described in the following.

The figure shows a functional block diagram of a system for determiningbrake lining wear and the rotational speeds of the wheels of a motorvehicle.

Reference numbers 11 a-d designate the wheel units of a motor vehicle.These wheel units include specifically the wheels, the peripheralvelocities (rotational speeds) of which are to be measured, and thebrake systems (friction brakes), one of which is assigned to each wheelunit. Reference numbers 102 a-d designate the rotational speed and brakelining wear sensors assigned to each wheel, which, insofar as theinvention is concerned, are described in greater detail on the basis ofFIGS. 2 and 3. With respect to the aspects of the design of thesesensors which goes beyond the scope of the invention, explicit referenceis made to the state of the art cited above.

The output signals of the rotational speed and brake lining wear sensors102 a-d are connected to a control unit 103; the transmission lines aredesignated 105 a-d. The data transmitted by means of transmission lines105 a-d are then evaluated centrally for all wheel units in control unit103. The status of the brake linings, as the evaluation result, is sentby control unit 103 over lines 18 a-d to a display instrument 110. Forthis purpose, it is provided in general that the driver is informedappropriately whenever the wear of one or more brake linings hasexceeded a certain value.

For the sake of completeness, reference numbers 14 a-d designate thebrake systems of the individual wheel units 11 a-d, which can beactuated by control unit 103.

FIGS. 2 and 3 show various embodiments on the basis of a single wheelunit by way of example.

FIG. 2 shows a simple combination of an active rotational speed sensorwith a brake lining wear detection function. As already mentioned above,a known Hall rotational speed sensor or a known magnetoresistiverotational speed sensor can be provided as “active” rotational speedsensor 102. It can be seen schematically in FIG. 2 that a sensor element1021 samples an incremental rotor 101 of a magnetically passive type.Depending on the sampled increments of rotor 101, two current levels i₁and i₂ are transmitted by sensor element 1021. This is shown in FIG. 2as the turning on and off of two current sources 1022, 1023.

Speed sensor 102 is connected to control unit 103 by lines 105 and plugconnections 1021 a, b and 1031 a, b. With the help of input resistor R,input amplifier 1036 detects the voltage values corresponding to thecurrent levels of rotational speed sensor 102:

U_(LOW)=R*i₁

U_(HIGH)=R*(i₁+i₂)

The lower signal (301) in FIG. 4 is typical of the course of the signalproduced by a wheel with an essentially constant rotational speed. Thedesired value, i.e., the rotational speed of the wheel, is obtained byevaluating the frequency of this signal.

The lower half of FIG. 2 shows in schematic fashion a known detector1104 of brake lining wear in a wheel brake. As already mentioned above,the brake lining wear sensor known in and of itself according to thestate of the art determines the wear of the brake lining of a vehiclebrake by the use of contact pins, for example, which are embedded in thebrake linings a certain distance below the surface. These pins trigger acontact when the brake lining has been worn down to this depth. In FIG.2, this contact is designated as switch 1041. In the normal case (brakelining wear not severe enough to warrant a warning), switch 1041 isopen, and voltage U+ is not grounded. Once the brake lining has worndown to a certain extent, switch 1041 closes, which is detected inevaluation circuit 1037 because of the grounding by connection 106 orplug connections 1021, 1031.

As can be seen from the embodiment shown in FIG. 2, separate signallines 105, 106 are required to transmit the wheel speed information andthe information concerning the status of the brake lining.

The system according to the invention is now to be explained on thebasis of FIG. 3. Rotational speed sensor 102 described in conjunctionwith FIG. 2 has been supplemented here by an additional current source1023, which is wired in parallel with the rotational speed sensor shownin FIG. 2. Another additional current source 1024 can be connected tothe circuit between the rotational speed sensor and the evaluation unitthrough transistor 1025 by means of connections 1027, 1026.

Transistor 1025 is driven by a signal S, which comes from switch 1041,already described on the basis of FIG. 2. Switch 1041 changes itsswitching status when it has been detected that the brake lining hasworn down to a certain degree.

In FIG. 3a, the rotational speed sensor described on the basis of unit102 in FIG. 2 and the additional current source 1024 and transistor 1025are shown combined into a unit 102′. When transistor 1025 is switched tothe conducting state, current i₃ is added to current levels i₁ and(i₂+i₂) described above. This produces the new, increased current levels(i₁+i₃) and (i₁+i₂+i₃), which are sent to control unit 103′.

Depending on the switching status of transistor 1025 and thus of switch1041, input amplifier 1036′ detects the voltage values corresponding tothe current levels mentioned above, with the help of input resistor R:

U_(LOW)=R*i₁

U_(HIGH)=R*(i₁+i₂)

or

U_(LOW)′=R*(i₁+i₃)

U_(HIGH)′=R*(i₁+i₂+i₃)],

as a function of whether excessive brake lining wear has been detectedor not.

In addition to typical curve 301 previously described for the case inwhich additional current source 1024 is turned off, the type of signalobtained with current source 1024 turned on can be seen in the uppercurve 302 of FIG. 4. Upper signal curve 302 is thus shifted with respectto lower curve 301 by the offset (R*i₃).

The desired rotational speed N of the wheel is obtained by evaluation ofthe frequency of these signals, shown as signal curves 301 and 302, inblock 1034 of FIG. 3b. This rotational speed N can then be sent to theactual brake controller, drive unit controller, or other open orclosed-loop controller. In the case of an open or closed-loop controllerfor the brakes or the drive unit, wheel brakes 11 a-d are actuated(signals 14 a-d) as a function of the detected rotational speeds.Frequency evaluator 1034 is designed in such a way that the frequenciesof signal curves 301, 302 are determined independently of theabove-cited offset produced by the switch position of switch 1041. As aresult of, the rotational speed will always be detected, i.e.,regardless of whether or not excessive brake lining wear is detected.This is important in terms of system availability.

Signals 301, 302 are sent not only for evaluation 1034 mentioned abovefor evaluation of the rotational speeds of the wheels but also for athreshold comparison 1032. This threshold comparison 1032 serves todetermine whether or not the offset (R*i₃) caused by the position ofswitch 1041 is present. On the output side of threshold comparison 1032there is therefore a signal available which provides information onwhether or not excessive brake lining wear is present. This informationcan be sent via display 110 to the driver of the vehicle.

It can be provided in particular that threshold comparison 1032represents a standard test of the input thresholds of an input amplifierto see if a short circuit is present. This test responds to an increasein the conventional signal 301 (signal without recognized brake liningwear) by the offset (R*i₃).

This embodiment has the advantage that no changes need to be made to thehardware of the control unit or its input amplifier in comparison with asystem without brake lining wear detection. It must be guaranteed,however, that it is possible for the system to distinguish betweenrecognized brake lining wear [offset (R*i₃)] and a short circuit.

This can be achieved by providing the input amplifier with a test mode.By means of this test mode, the software in the control unit can tellwhether a “genuine” short circuit is present or whether brake liningwear has been detected. In addition, it is extremely unlikely that brakelining wear would occur simultaneously at all wheels of the vehicle.

As already mentioned, the rotational speed sensor described in FIG. 3aon the basis of unit 102 in FIG. 2, additional current source 1024, andtransistor 1025 are shown combined into a unit 102′. The particular goalhere is to combine the rotational speed and brake lining wear detectionfunctions by means of an integrated type of design. Signal S of switch1041 located near the brake lining will then be sent to a component suchas this, which will usually be installed near the rotational speedsensor, i.e., in the area of incremental rotor 101.

FIG. 5 shows a second design variant of the invention. Here referencenumber 502 designates a unit which, like unit 102′ described above,combines the actual rotational speed detector and some of the componentsused to determine brake lining wear. Unit 502 is connected by way oflines 5051, 5052 to inputs 1031 a, 1031 b of a control unit, not shownin FIG. 5. This control unit is explained further below in greaterdetail on the basis of the block circuit diagram of unit 103′ shown inFIGS. 3a and 3 b. In addition, unit 502 is connected by terminals 5053,5054 to a brake lining switch S1 (corresponding to switch 1041 of FIGS.2 and 3a). In this exemplary embodiment, switch S1 is closed in thenormal case (brake lining wear not severe enough to warrant a message).

The actual detection of the rotational speed is achieved in a mannersimilar to that described on the basis of FIGS. 2 and 3a. Namely, an“active” rotational speed sensor, e.g., a known Hall rotational speedsensor or a known magnetoresistive rotational speed sensor is provided.It can be seen schematically in FIG. 2 in this regard that a sensorelement 1021 samples an incremental rotor 101 of a magnetically passivetype. Depending on the sampled increments of rotors 101, sensor element1021 sets two current levels i₁ and i₂. This is shown in FIG. 2 as theturning on and off of current source i₂, i.e., 1022. FIG. 5 shows adifferent view of the same rotational speed detection system.Incremental rotor (101 in FIGS. 2 and 3a) is not shown here, and thesensor element (1021 in FIGS. 2 and 3a) is designated by referencenumbers 5030, 5031. Current source i₂ (high level) is turned on here bythe switching of transistor 5032. Block 5030 is intended to represent asensor circuit known in and of itself in the form of a Wheatstonebridge, by means of which current source i₂ is turned on or off byoperational amplifier 5031 and transistor 5032. By means of theevaluation described on the basis of FIGS. 2 and 3 a-b, the rotationalspeed signal is thus obtained, as shown by the lower signal curve inFIG. 4 and also in FIG. 6a. The essence of this design variant is thedesign of the expanded brake lining wear detector (expanded BLWdetector) 502 a.

When the brake lining has been worn down to a certain degree, switch S1opens. When switch S1 is opened, a low level signal passes to the upperinput of a logical AND gate 5021 shown in FIG. 5; when switch S1 isclosed, this input of AND gate 5021 has a high level. The drive signalof transistor 5032 is sent in inverted form to the lower input oflogical AND gate 5021. This means that, when transistor 5032 is actuated(current source i₂ turned on, rotational speed signal at high level), alow level (inverted) is present at logical AND gate 5021. When currentsource i₂ is turned off by the transistor (low level at transistor5032), a high level is present at the lower input of AND gate 5021 as aresult of the inversion. There is then a high level present at theoutput side of AND gate 5021 when the degree of brake level wear doesnot warrant a warning display (switch S1 closed, upper input of the ANDgate at high level) and simultaneously current source i₂ is turned off.Otherwise, the output of the AND gate is at the low level.

The output of AND gate 5021 is connected to the input of logical OR gate5022. Comparators K1 and K2, furthermore, are connected to the other twoinputs of OR gate 5022.

Comparator K1 compares the input voltage V_(CC) of sensor unit 502 witha presettable threshold value REF.K1. This is done to detectundervoltages, which can impair the proper operation of unit 502. Ifsuch an undervoltage occurs, that is, if V_(CC) is smaller than REF.K1,a high level is applied to the upper input of OR gate 5022. Otherwise,this input remains at the low level.

Comparator K2 compares the temperature detected by temperature sensor5025 with a presettable threshold value REF.K2. Temperature sensor 5025measures the temperature to which sensor unit 502 is exposed.Temperature sensor 5025 can be, for example, a diode, thetemperature—dependent forward voltage of which is measured, and can beintegrated in a manner known in and of itself directly into integratedcircuit (IC) of sensor unit 502. The background of this temperaturemeasurement is that sensor unit 502 is usually mounted close to thewheel and therefore close to the brake disks as well. The heat emittedby the brake disks can heat up sensor unit 502 to such a degree that theproper operation of unit 502 can be impaired. If such overheatingoccurs, i.e., if the measured temperatures are greater than REF.K2, ahigh level is applied to the lower input of OR gate 5022. Otherwise,this input remains at the low level.

On the output side of OR gate 5022, there is therefore a high signalwhen at least one of the three inputs is at the high level, when,therefore:

either sensor unit 502 is overheated or

an undervoltage is present or

brake lining wear severe enough to warrant a warning message is notpresent and simultaneously current source i₂ is turned off.

Otherwise, the output of the OR gate is at the low level.

The output of OR gate 5022 is connected to a reset input R of a counter5023. Counter 5023 is set back when there is a high signal at the Rinput. Clock input C of counter 5023 is connected to the drive signalfor transistor 5032. Input C thus receives a high level when currentsource i₂ is turned on and a low level when current source i₂ is turnedoff. Counter 5023, which is designed in a manner known in and of itselfas a flip-flop switch, is therefore always switched when current sourcei₂ is turned on or off. Counter 5023 has three outputs, which have ahigh level when the level at clock input C has changed the first,second, and fourth time from low to high. In this way it is achievedthat, when current source i₂ has been turned on the fourth time, threehigh levels are present at AND gate 5024, to which the outputs ofcounter 5023 are sent. In this case (all three inputs of AND gate 5024at high), the AND gate also supplies a high level on the output side,whereupon the third current source i₃ is turned on. Current i₃ ofcurrent source i₃ is then superimposed on the current (i₁+i₂) present atthis time, which leads to a total current of (i₁+i₂+i₃) at output 5052.The turning on of current source i₃ can occur by means of a transistor,not shown in FIG. 5, connected in series with this current source i₃.This would be done in a manner similar to that shown in FIG. 3a, wherecurrent source i₃ is turned on and off by transistor 1025.

FIG. 6a shows the signal present at output 5052 when switch S1 is closed(brake lining wear not severe enough to warrant a warning message). Theupper input of AND gate 5021 shown in the lower signal curve of FIG. 6ais then set to high. By means of OR gate 5022, counter 5023 (input R) isalways reset when current source i₂ is turned off. In this way, it isensured that the third current source i₃ does not remain turned off whenthe brake lining wear is not severe enough to warrant a warning message.In control unit 103′ (input 1031 b), the signal present at output 5052is then converted by way of resistor R into a voltage, whereupon, bymeans of the frequency analysis 1034 already described, the rotationalspeed N of the wheel is determined.

FIG. 6b shows the course of the signal present at output 5052 whenswitch S1 is open (brake lining wear severe enough to warrant a warningmessage). The upper input of AND gate 5052 shown in the lower signalcurve of FIG. 6b is then set to low. By means of OR gate 5022, counter5023 (input R) is reset only when an undervoltage (comparator K1) or anexcessive temperature (comparator K2) is present. In the normal case(neither undervoltage nor excessive temperature) input R of counter 5023remains at low, whereupon, every fourth time current source i₂ is turnedon, current source i₃ is turned on. This produces the rotational speedsignal shown in the upper part of FIG. 6b. In control unit 103′ (input1031 b), the signal present at output 5052 is then converted by way ofresistor R into a voltage, whereupon, by means of the previouslydescribed frequency analysis 1034, the rotational speed N of the wheelis determined. In addition, threshold comparison 1032 determines whetheror not level R*(i₁+i₂) has been exceeded. In the case of brake liningwear severe enough to warrant a warning message, this is based on theincrease in the fourth high level of the rotational speed signal and isdisplayed by display means 110.

By way of example, FIG. 7 shows how, according to another design variantof the invention, an excessive distance between a Hall ormagnetoresistive sensor and the previously described toothed wheel rimof the vehicle wheel, the rotational speed of which is to be detected,can be determined. Sensor element 5030 is the sensor element designatedby the same reference number in FIG. 5. As already mentioned in thatcontext, element 5030 is a known Wheatstone bridge with a typicalring-shaped arrangement of resistors. As the individual segments of thetoothed wheel rim (not shown) pass by, a bridge voltage U_(B) isproduced in this Wheatstone bridge, and this voltage is sent tocomparators 5031, 5101. Comparator K1 corresponds to the comparatordesignated by the same reference number in FIG. 5 and serves to evaluatethe rotational speed of the wheel. Comparator K2 5101 evaluates thebridge voltage in another way; that is, the bridge voltage is comparedwith a relatively high threshold value U_(H). The background of the twothreshold comparisons will be discussed in the following in connectionwith FIG. 8.

FIG. 8 shows a typical signal course of the bridge voltage over time.Depending on the rate at which the individual segments of the toothedwheel rim pass by, the bridge voltage increases and decreasesperiodically. If the distance, i.e., the air gap, between the toothedwheel rim and the Wheatstone bridge 5030 remains constant, the bridgevoltage has a constant amplitude. If this distance become too large,however, the amplitude of the bridge voltage decreases. This case isshown in FIG. 8.

A first threshold comparison in comparator 5031 compares the bridgevoltage signal with a relatively low threshold value such as 40 mV. Onthe output side, comparator 5031 supplies the drive signal for currentsources i₁ and i₂, shown by the lower signal curve K1 in FIG. 8 (seeFIG. 5). Signal K1 therefore represents the rotational speed of thewheel, even when the air gap is increasing. Comparator 5101 also checksthe amplitude of the bridge voltage signal, but in this case arelatively high comparison threshold of 60 mV, for example, is used. Ifthe distance between the toothed wheel rim and the Wheatstone bridge,i.e., the air gap, is sufficiently small, the amplitude of the bridgevoltage signal is above the threshold of comparator 5101. As can be seenfrom the lower signal curve K2 in FIG. 8, under proper conditions theoutput signal of comparator 5101 is characterized by a time delay insignal K2 with respect to signal K1. But if the signal from comparatorsignal K2 disappears completely, this means that the amplitude of thebridge voltage signal has decreased and that the air gap is too large.

The absence of signal K2 is detected in unit 5102, where an absence ofsignal K2 results in the generation of a signal LS.

In summary, it can be said with respect to the detection of the air gapthat an active sensor such as a Hall sensor or a magnetoresistive sensoris used to detect the rotational speed signals of a wheel. The sensorscontain a Wheatstone bridge, which is detuned by changes in the magneticfield. The signal for the rotational speed is obtained from thisdetuning. The extent of the detuning stands in a fixed relationship tothe magnitude of the differences between the magnetic fields of the twohalves of the bridge. The magnetic field difference depends on, amongother things, the distance between the sensor and the pole wheel. Byevaluation the extent of the bridge detuning, it is possible to draw aconclusion concerning the size of the air gap between the sensor and thepole wheel. This evaluation can be carried out with comparator 5101,which has a greater hysteresis (U_(H)=60 mV) than normal useful-signalcomparator 5031 (U_(H)=40 mV). If the air gap is small, both comparatorsconnect through, but if the air gap is too large, only useful-signalcomparator 5031 connects through. In this way, an early warning systemfor an excessive air gap is obtained, without any loss of datapertaining to the rotational speed of the wheel. This information can beused, for example, for end-of-line monitoring during the manufacture ofmotor vehicles, in the repair shop, or while driving.

FIGS. 9a and 9 b show by way of example the evaluation carried out toidentify the direction of rotation of a wheel. For this purpose, FIG. 9ashows a Hall or magnetoresistive sensor 5030′, which has been modifiedwith respect to the sensors shown in FIG. 7 and FIG. 5. The modificationconsists in that the known Wheatstone bridge, as can be seen in FIG. 9a,is expanded by two additional resistors. Instead of this modifiedWheatstone bridge, it is also possible for a modified Hall ormagnetoresistive sensor to have two separate sensing elements 50301′ and50302′ or to provide two complete Wheatstone bridges (FIG. 9b). Here,too, the individual elements of the toothed wheel rim, pole wheel, ortransmitter wheel (101, FIGS. 2, 3 a, and 9 b) to produce correspondingchanges in the bridge voltage signals U_(B1) and U_(B2). These bridgevoltage signals are sent to evaluation unit 5201. Simultaneously, atleast one of the bridge voltage signals is sent for evaluation of theuseful signal to comparator 5031, described previously (FIGS. 5, 7, 8).The function of rotational direction detector 5201 is explained below onthe basis of FIGS. 10a and 10 b.

FIGS. 10 and 10b show the course of the bridge voltage signals of thebridge shown in FIG. 9a. Either the change over time t, the change overdistance s, or the change over the rotational angle of the transmitterwheel can be considered. Depending on the direction in which the wheelis rotating, either the right part of modified Wheatstone bridge 5030′is detuned first or the left part. When the wheel is turning toward theright, bridge voltage U_(B1) precedes bridge voltage U_(B2), whereas thesituation is reversed when the wheel is rotating toward the left.Rotational direction evaluator 5201 evaluates the phase shift of the twobridge voltage curves, and a signal DR is generated when the wheel isturning backwards. As an alternative, it is also possible, as can beseen in FIG. 10b, for the difference ΔU_(B) between the two bridgevoltage values U_(B1) and U_(B2) to be formed. The information DRconcerning the direction of rotation (forwards/backwards) is obtainedfrom the change in this difference U_(B), especially from the positionsof the maxima and minima (peaks pointing “up” or “down”).

FIG. 11 shows an expansion of FIG. 5. Whereas FIG. 5 has as its objectthe transmission of the rotational speed signal of the wheel and thebrake lining wear signal to control unit 103, the goal of the expansionin FIG. 11 is to transmit a rotational direction signal DR, an air gapsignal LS, and a brake lining wear signal BLW. For this purpose, as theoutput signal of unit 5022 (FIG. 5), the brake lining wear signal BLW,which represents excessive brake lining wear, is sent as input tocounter 5023′, 5024′. The output signal LS of air gap detector 5102 issent as input to counters 5023″, 5024″, whereas output signal DR ofrotational direction detector 5201 is sent as input to counters 5023′″,5024″′. The three counters are synchronized by the output signal ofcomparator 5031 (FIG. 5). What occurs therefore is that the counters aresynchronized with the rotational speed of the wheel.

The three counters shown in FIG. 11 differ in that they carry out adivision by 8, by 4, and by 2. On the output side, all 3 counters areconnected to the input of logical OR gate 1101. The additional currentsource i₃ (FIG. 5) is driven by the output of OR gate 1101. FIG. 13 mustnow be described in order to show how the circuit in FIG. 11 functions.

In the upper part of FIG. 13 (signal 1), we see the wheel rotationalspeed signal normally present at output 5052 (FIG. 5) of the sensorelement, that is, the signal without any additional informationsuperimposed on it. If now, according to FIG. 5, for example, anexcessive degree of brake lining wear is detected (unit 5022) signal BLWcauses every eighth high level of the wheel rotational speed signal tobe increased by means of the counters 5023′, 5024′. This can be seen inthe second signal of FIG. 13. If the air gap is too large, every fourthhigh level is increased by means of the corresponding counter, as shownin the third signal n FIG. 13. In the same way, the detection ofbackwards travel brings about an increase of every second high level ofthe wheel rotational speed signal (fourth signal in FIG. 13). The wheelrotational speed signal (signal 1 in FIG. 13) or the modified wheelrotational speed signal (signals 2, 3, 4 in FIG. 13) are present at theoutput 5052 (FIG. 4) of sensor element 502. This signal is sent tocontrol unit 103, where the evaluation shown in FIG. 12 takes place.

Via input plug 1031 b, the input signal is converted via precisionresistor R into a corresponding voltage value. In a first comparatorK10, the signal is compared with a relatively low threshold value SW1.This threshold value, as can be seen in FIG. 13, is selected to be solow that it is exceeded by the high level of the normal (not increasedto the high level) rotational speed signal (i₁+i₂), i.e., itscorresponding voltage value. The wheel rotational speed signal is thenpresent at output A independently of the increase in the high levelaccording to the invention and can be evaluated in frequency evaluator fto obtain the rotational speed N of the wheel. Simultaneously, thevoltage values are also sent to comparator K11, where the voltage signalis compared with a relatively high threshold value SW2. As can be seenin FIG. 13, only a high level which has been increased according to theinvention exceeds this high threshold value SW2. Depending on thepresence of information (backwards travel, air gap too large, excessivebrake lining wear), we thus arrive at signals 2B (evaluation of thesecond signal curve), 3B (evaluation of the third signal curve), and 4B(evaluation of the fourth signal curve), shown in FIG. 13, at the outputof comparator K11. Signal curves B are evaluated in evaluation unit1039.

In regard to evaluation unit 1039, it should be remarked that, in thepresence of backwards travel (signals 4, 4B in FIG. 13), every secondhigh level is raised. We thus arrive at a signal 4B, which has half thefrequency of rotational speed signal A. By a comparison of output signalB of comparator K11 with rotational speed signal A of the wheel inevaluation unit 1039, the conclusion is drawn that, for example, in thecase shown in FIG. 13 by signal 4B, the vehicle is traveling backwards.This information can be made available as an output signal of evaluationdevice 1039 and used for a wide variety of additional data processingapplications. Information of this type with respect to backwards travelis extremely helpful to so-called “hill holders”, for example, andnavigation systems.

Whereas backwards travel can be clearly detected, the embodiment doesnot make it possible to determine, while the vehicle is travelingbackwards, whether or not an excessive air gap and/or excessive brakelining wear is present. Only after the vehicle has stopped travelingbackwards and signal curve 3B, for example, is present at evaluationunit 1039, can it be concluded, by means of an appropriate frequencycomparison with rotational speed [signal] A, that either an excessivedegree of brake lining wear or an overly large air gap is present.Because a vehicle usually travels backwards for only a relatively shortperiod of time, whereas an excessive air gap or an excessive degree ofbrake lining wear occurs simultaneously and spontaneously only in therarest of cases, the superimposition of the information in this designvariant does not represent a problem. After backwards travel has ended,a comparison of wheel rotational speed frequency A with the frequency ofsignal 2B will reliably determine whether or not an excessive degree ofbrake lining wear is present. If signal 3B occurs, however, then again afrequency comparison will show whether or not an excessive air gap ispresent.

The basis of the exemplary embodiments described so far has been thedetection of the rotational speed of a wheel of a vehicle (as alreadymentioned, the invention can also be used to determine the rpm's of anengine), where at least two current levels i₂ and i₂ are generated bymeans of a so-called “active” sensor. At least one of these currentlevels is then changed by a third current level i₃ so that additionaldata (brake lining wear, air gap, direction of rotation) can betransmitted. In these variants, the current levels are transmitted fromsensor unit 502 (FIG. 5) to control unit 103′ (FIG. 3). This offers theadvantage that only a two-wire connection is required between sensorunit 502 and control unit 103′. Nevertheless, these variants lead to acertain power dissipation, especially when the third current source i₃is turned on.

Against this background, a variant is described in the following, inwhich the information, especially the rotational speed and direction ofrotation, are sent by different voltage levels from sensor unit 502 tocontrol unit 103′. Although a three-wire connection (voltage supply,ground, signal line) is required between sensor unit 502 and controlunit 103′, this variant has certain advantages with respect to the powerdissipation mentioned.

FIG. 14 shows a sensor unit 140, in which a rotational speed signal Nand a rotational direction signal DR are generated by means ofpreviously described sensor 5030′ (FIGS. 9 and 10), comparator 5031(FIGS. 5, 7, and 9 a), and rotational direction detector 5201 (FIGS. 9and 10). These two signals are shown at the top 2 in FIG. 16. Bothrotational speed signal N and rotational direction signal DR have twovoltage levels “0” and “1”; the frequency of rotational speed signal Nindicates the rpm's of a wheel of the vehicle, the rpm's of the engine,or the rpm's of a transmission. Rotational direction signal DR has thevoltage level “0” when the vehicle is traveling forwards or when theengine of the vehicle is rotating in the normal direction. Therotational direction signal DR has the voltage level “1” when thevehicle is traveling backwards or when the vehicle engine is rotating inreverse.

These signals are processed in unit 1401 by means of logical AND gate14013 and inversion stage 14011, so that signals N_(V) and N_(R) arepresent at the output of unit 1401. The course of these signals can beseen as signal curves 3 and 4 in FIG. 16.

Signals N_(V) and N_(R) now drive transistors 1402, 1403 in such a waythat a “0” level of the N_(V) signal blocks transistor 1403 or a “1”level of the N_(V) signal switches transistor 1403 to the conductivestate. Transistor 1402 is blocked by a “0” level of the N_(R) signal andswitched to the conductive state by a “1” level of the N_(R) signal.While transistor 1402 is in the conductive state, voltage level “0” ispresent on the output side of sensor unit 140, on signal lineU_(signal), independently of the switching status of transistor 1403. Iftransistor 1402 is blocked, then, depending on the rotational speed, thevoltage levels V_(CC) and V_(CC)/2 are present on the output side ofsensor unit 140 on signal line U_(signal), as a function of theswitching position of transistor 1403, under the assumption thatresistors R1 have the same rating. Level V_(CC) comes about in that,when transistors 1402 and 1403 are blocked simultaneously, no morecurrent flows and therefore there is no voltage drop at R1.

In summary, the following switching table describes the operation ofsensor unit 140 (FIGS. 14 and 16):

DR N N_(R) N_(V) U_(Signal) 0 0 0 1 VCC/2 0 1 0 0 VCC 1 0 1 1 0 1 1 0 0VCC

where:

N=the rotational speed (square-wave signal from the processing circuit);

DR=the rotational direction (DR=0: forwards, DR=1: backwards, from theprocessing circuit);

NR=rotational speed square-wave signal for driving transistor 1402(active only during backwards rotation, otherwise 0);

N_(V)=rotational speed square-wave signal for driving transistor 1403(always active); and

U_(signal)=output signal of sensor unit 140.

FIG. 15 shows how signal U_(signal), generated in sensor unit 140, isevaluated. Supply voltage V_(CC) is provided by voltage supply 1505.Signal U_(signal) (signal 5 in FIG. 16) is sent to two comparators 1503,1504, where the voltage levels are compared with two threshold valuesSW1, SW2. These two threshold values are drawn on signal 5 of FIG. 16.As comparison results, signals S1, S2 are available on the output sideof comparators 1503, 1504; these result signals are shown as signal 6and 7 in FIG. 16. The frequency of signal S1 can be evaluated byfrequency evaluator 1501 to determine the rotational speed, whereas itis determined in unit 1502 whether signal S2 has reached the high level,from which it can be concluded that the vehicle is traveling backwardsor that the engine of the vehicle is rotating backwards (informationDR′).

Rotational speed signal N′ and rotational direction signal DR′ aresubjected to further processing in control unit 150.

At this point it should be pointed out that, in principle, therotational direction signal can also be encoded. That is, instead oflowering every low level of the rotational speed signal in the event ofbackwards rotation (signal 5, FIG. 16), it is possible, in analogy toFIG. 13 previously described, to lower or raise every n-th signal level.

Because, in cases where it is in fact possible to detect the backwardsrotation of an internal combustion engine, the idea is to detect thiscondition as quickly as possible, it is advantageous in this applicationfor every level to be changed immediately once backwards rotation hasbeen determined.

What is claimed is:
 1. A system for producing a signal representing a rotational movement of a wheel on a motor vehicle having an engine, said system comprising: a first means for generating a first signal representing the rotational movement and said first signal assuming at least two first values of current or voltage; a second means capable of generating at least one second signal said second signal representing one or more of: brake lining wear in at least one wheel brake of the vehicle, the direction of said rotational movement, the amplitude of a value associated with the first signal, other operating conditions of the vehicle wheel, other operating conditions of the vehicle brake, and other operating conditions of the vehicle engine; and a third means capable of changing at least one of the first values of the first signal, wherein the occurrence of said change is dependent upon the generation of said second signal.
 2. A system according to claim 1, wherein the system is part of a motor vehicle, and the first signal represents a rotational speed selected from the group consisting of rotational speed of a vehicle wheel; rotational speed of a motor of the vehicle, said motor being a gasoline or diesel engine or an electric motor; and rotational speed of a shaft operationally connected to a transmission of the vehicle.
 3. A system according to claim 1, wherein the first means comprises an active rotational speed sensor, and the third means changes the first signal by raising at least one of the first current values for a period of time to a second current value as a function of the second signal.
 4. A system according to claim 1, wherein the second means generates signals which represent at least said brake lining wear in at least one wheel brake of a vehicle, said direction of rotational movement, and said amplitude of a value associated with the first signal.
 5. A system according to claim 3, wherein the third means has a current source.
 6. A system according to claim 5, wherein the third means includes switching means for turning on and off the superimposition of at least one of the two first current values onto a current induced by the current source.
 7. A system according to claim 5, wherein the third means includes switching means for turning the current source on and off.
 8. A system according to claim 6 wherein the switching means has a first switch and a second switch, preferably designed as a transistor, and the first switch is located near a brake lining and the second switch is located near the rotational speed sensor.
 9. A system according to claim 8, wherein the first switch has a switching status dependent on the degree of brake lining wear, and the second switch has a switching status dependent on the switching status of the first switch.
 10. A system according to claim 3, wherein the active rotational speed sensor for generating the at least two first current values has at least two current sources.
 11. A system according to claim 1, wherein a means of transmission transmits the first signal or the changed first signal to an evaluation means for evaluating the rotational movement.
 12. A system according to claim 11, wherein, the evaluation means has conversion means which converts the current values of the first signal into corresponding voltage values.
 13. A system according to claim 12, wherein, the evaluation means performs at least one threshold value comparison comparing the current values or the corresponding voltage values with at least one threshold value result, and wherein display means can be driven as a function of the comparison result.
 14. A system according to claim 1, wherein the number of times the change of the first signal by the third means dependent on the second signal occurs is determined so that at least one of the two first current values is increased after said change has occurred a defined number of times.
 15. A system according to claim 14, wherein the defined number of occurrences is selected as a function of the type of information to be transmitted in the second signal, said type of information to be transmitted in second signal including at least one the direction of rotation, excessive air gap or excessive brake lining wear.
 16. A system according to claim 15, wherein the first signal assumes the two first current values periodically, and at least one of the two first current values is increased after the first signal has assumed said current value n times, where n is a number equal to or greater than one.
 17. A system according to claim 16 wherein the number n is dependent on the signal generated by the second means.
 18. A system according to claim 2, wherein the third means changes the first signal in a way which is carried out as a function depending on at least one signal of the group consisting of: a signal representing the temperature of a wheel brake of the vehicle; and a signal representing the temperature of a rotational speed sensor; and a signal representing a supply voltage (V_(CC)) of the rotational speed sensor.
 19. A system according to claim 11, wherein the system is part of a motor vehicle; wherein the first signal represents the rotational speed of the vehicle motor, said motor vehicle is a gasoline and/or diesel engine; wherein the second means generates a signal representing the rotational direction of the vehicle's engine; and wherein the evaluation means is an engine control unit which generates ignition and/or injection signals and which suppresses at least individual ignition signals in cases where it has been recognized that the vehicle's engine is rotating backwards.
 20. A system according to claim 7 wherein the switching means has a first switch and a second switch and the first switch is located near a brake lining and the second switch near the rotational speed sensor.
 21. A system according to claim 20 wherein said switches are transistors.
 22. A system according to claim 8 wherein said switches are transistors.
 23. A system for producing a signal representing a relative movement between a body of a vehicle and a wheel unit of said vehicle, said system comprising: a first means for generating a first signal representing the movement between the body of the vehicle and a wheel unit of the vehicle relative to one another; said first signal assuming at least two first values of current or voltage; a second means for generating at least one second signal representing additional information, said second signal containing information representing whether the vehicle body is moving toward or away from the wheel unit; and a third means for changing at least one of the first values of the first signal, for a certain period of time, as a function of said second signal. 